Geochemical Evolution of Brines in Salar of Uyuni, Bolivia

Chemical Geology, 30 (1980) 57--79
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
57
G E O C H E M I C A L E V O L U T I O N OF BRINES IN THE S A L A R OF UYUNI,
BOLIVIA
S.L. RETTIG 1 , B.F. JONES I and F. RISACHER2
U.S. Geological Survey, Reston, VA 22092 (U.S.A.)
2Mission ORSTOM, Cajon Postal 8714, La Paz (Bolivia)
(Received November 13, 1979; accepted for publication April 1, 1980)
ABSTRACT
Rettig, S.L., Jones, B.F. and Risacher, F., 1980. Geochemical evolution in the Salar of
Uyuni, Bolivia. Chem. Geol., 30: 57--79.
Recent analyses of brines from the Salars of Uyuni and Coipasa have been compared
with published chemical data for Lakes Titicaca and Poopo in order to evaluate solute
compositional trends in these remnants of two large Pleistocene lakes once connected
by overflow from the northern to the southern part of the Bolivian Altiplano. From
Titicaca to Poopo the water chemistry shows an increase in concentration of CI and Na
somewhat greater than the total solutes, suggesting preferential inputs of these elements
or significant losses of other constituents downstream from Lake Titicaca proper. In contrast, Ca and SO 4 increase to a lesser extent than do total dissolved solids, and carbonate
species are relatively constant, suggesting solute losses most readily related to the precipitation of calcite and gypsum. Between Poopo and Coipasa the proportions of Ca, SO 4 and
CO 3 continue to decrease. At Coipasa and Uyuni, the great salars frequently evaporate
to halite saturation and widespread halite crystallization is accompanied by an increased
proportion of K, Mg and SO4 in residual brines.
Notably high concentrations of Li and B in brines from the south end of Uyuni, near
the mouth of the Rio Grande de Lipez, are clearly in excess of that expected for the solute
concentration trends for inflow from Coipasa and the northern Bolivian Altiplano, as shown
by comparison with Mg and K. This supports other indications that the major source of Li
and B are the recent rhyolitic volcanic rocks prevalent in the drainage of the Rio Grande.
INTRODUCTION
T h e Salar de U y u n i , in t h e s o u t h w e s t e r n c o m e r o f Bolivia (Fig. 1) is t h e
largest o f m o r e t h a n 75 salt pans o r playas w h i c h lie in t h e closed basins o f
the central A n d e a n A l t i p l a n o at altitudes o f m o r e t h a n 3 6 0 0 m. U y u n i , w h i c h
has an area o f a b o u t 9 0 0 0 k m 2, is in f a c t t h e largest salt fiat in t h e world. Alt h o u g h f o r m a n y years salt has b e e n m i n e d at U y u n i in sufficient q u a n t i t i e s
t o s u p p l y all o f Bolivia, o n l y q u i t e r e c e n t l y was t h e r e a n y scientific discussion
o f its brine resources (Ericksen et al., 1976, 1 9 7 8 ; Risacher and Miranda,
1 9 7 7 ) . Ericksen et al. ( 1 9 7 6 ) suggested t h a t t h e Bolivian salars m i g h t c o n t a i n
a significant a c c u m u l a t i o n o f Li. In S e p t e m b e r o f 1 9 7 6 a field p a r t y f r o m
58
70o
68 o
16 o
18 G
20 t
Fig. 1. Outline map of the Bolivian Altiplano showing areal extent of present and ancient
lakes, and the principal inflow tributary to them.
GEOBOL .1 led by G.E. Ericksen and J.D. Vine made a brief systematic
sampling o f Uyuni and two nearby salars, Coipasa and Empexa. Over a similar period ORSTOM .2 (F. Risacher) sampled cross-sections in the area of
the Rio Grande de Lipez delta and adjacent salt crust on the south edge of
the Salar de Uyuni, some samples of which were analyzed in U.S.G,S. .3
laboratories. This paper discusses the results and implications of the chemical
analyses of these samples, and a few others collected subsequently from
Uyuni and southern tributary drainage by GEOBOL, ORSTOM and USGS.
GEOBOL: Servicio Geologico de Bolivia.
*20RSTOM: Office de la Recherche Scientifique et Technique Outre-Mer.
*3USGS: U.S. Geological Survey.
* z
59
HYDROLOGIC SETTING
According to Ahlfeld and Branisa (1960), in early Pleistocene times much
of the Bolivian Altiplano was covered by t w o large lakes: Lake Ballivian, in
the north, of which Lake Titicaca is a remnant, and Pre-Minchin in the south.
The latter lake was succeeded in the late Pleistocene b y Lake Minchin (3760 m,
15,000 yr. B.P.) and later b y Lake Tauca (3720 m, 10,000 yr. B.P.), which is
marked by well-developed terraces and deposits of algal limestone up to 75 m.
above the level of the Salar de Uyuni at 3653 m (Servant and Fontes, 19781.
Fig. 1 shows the extent of the ancient lakes and t h e system as it exists today.
The present area of Lake Titicaca is more than 8300 km 2 and its maximum
depth exceeds 300 m. Outflow from Lake Titicaca now drains southward to
Lake P o o p o through the Rio Desaguadero, which was the connection between
the Pleistocene lakes as well. According to H u e o t (1908), Lake P o o p o in 1905
covered an area of 3000 km 2 and had a total dissolved solids (TDS) content
of ~ 3.5 • 104 mg/1. Since that time it has fluctuated widely; in 1958 it was
reported to be completely dry (Ahlfeld and Branisa, 1960) and since then its
level has risen.
The water and solute budgets for Lakes Titicaca and P o o p o have been
evaluated for the period 1976--1977 b y Carmouze et al. (1978). Using a
chloride balance, they calculated that only 4.5% of the annual water loss
from Titicaca was outflow to the Rio Desaguadero; 90% was lost to evaporation and the rest to groundwater infiltration. This has resulted in an approximately five-fold concentration from inflow to effluent. At the same
time, the proportion of Na and C1 has increased to more than half of the
major solutes at Lake Titicaca; according to Carmouze et al. (1978) the chemical composition of the lake over their period of intensive sampling has varied
little from values obtained sporadically from 1956 to 1973. This can be attributed to a residence time of more than 600 years for the major cations
(Na, K, Mg) and chloride.
In contrast to Titicaca, Lake P o o p o has become a closed lake except for
groundwater infiltration at its south end. Carmouze et al. (1978) suggested
that 98% of the annual inflow to the lake was lost through evaporation during
their period of study. At the time, the lake surface was estimated at 2500 km 2,
with 60% of the area having a mean depth of ~ 1.5 m and the remainder of less
than 0.5 m. Carmouze et al. (1978) showed that TDS concentrated b y more than
thirty times over Lake Titicaca, that the residence time for Na and C1 was
just over 25 years, and that these solutes constituted 75% of the total dissolved
ions.
The Salar of Coipasa was at one time apparently connected to Lake P o o p o
through an ephemeral drainage way to the east, now referred to .as the l~io
Lacajahuira, b u t the salar is n o t presently receiving any new material from
that source. Coipasa is now supplied chiefly b y several rivers from the north,
creating a very shallow b o d y of brine, highly variable in an area at the north
end o f the salar.
60
The Salar de Uyuni, ~ 7 m lower than Coipasa, apparently has been a
closed basin since the last glacial period about 10,000 yr. B.P. Its only present
surface-water inflow is from the Rio Grande de Lipez to the south and lesser
drainages to the east. Uyuni is not as extensively flooded as Coipasa, but it
contains a large body of brine enclosed in a very porous NaC1 crust up to
10 m thick. During the rainy season the salar may be brine covered to a depth
of about 25 cm, but during the dry season the brine usually evaporates to the
salt surface or just below it. Evaporation from the brine "water table" in the
d r y season cements the upper crust pores. The annual flooding and evaporation gives Uyuni an exceptionally smooth surface with relief of only a few
centimeters. The region is semi-arid with an average annual rainfall of about 30
cm, and for the last several thousand years hydrographic conditions appear
to have been more or less stable.
The region of the great Altiplano salars themselves is underlain by Pliocene
and Quaternary volcanic rocks, including thick rhyolitic ash-flow tuffs,
especially to the south. Extinct or dormant volcanoes dot the area, and there
are a number of saline thermal springs. In times of greater volcanic activity, the
thermal springs were probably more numerous.
GEOCHEMICAL EVOLUTION OF WATERS FROM THE NORTHERN ALTIPLANO
Because the entire region has functioned as a single closed basin intermittently through the Pleistocene, we have begun with the premise that the geochemical evolution of waters on the Bolivian Altiplano can be compared to that
expected from the stepwise evaporative concentration of a single water mass.
The assumption that the chemistry of the major surface water bodies of the
southern Altiplano has a singular origin is inspired by consideration of the
total area (at least 43,000 km 2) covered by Pleistocene lake (see Servant and
Fontes, figs. 8 and 9). Fig.2 shows the major solutes plotted against TDS on a
log-log scale for the waters of Lakes Titicaca and Poopo and the most concentrated brines of the Salars of Coipasa and Uyuni, as compared to the 1:1
slope expected for evaporative concentration if the total solute concentration
is conserved. The values used for Lakes Titicaca and Poopo and the Rio
Desaguadero were recalculated from the average analyses of Carmouze et al.
(1978). The plots show that between the waters of Lake Titicaca, the Rio
Desaguadero, and Lake Poopo, there is an increase in dissolved C1 and Na
greater than the total dissolved solids content. On the other hand, Ca, Mg
and SO4 do not increase as rapidly as the TDS concentration. Without widespread and detailed analyses of lacustrine sediments from the area, it is not
possible to estimate how much of the apparent constituent gain or loss is due
to varying inflow composition, ion exchange, sorption, or mineral precipitation. Nevertheless, some attractive suggestions can be made.
Indications from the data of Carmouze et al. (1978) were that changes in
solute proportions due to different compositions of inflow waters to Lake
Poopo other than the Rio Desaguadero are minor. However, more recent
61
1,OOO,OOO
,
,
i,,,,
I
'
'
'1
....
I
'
'1
....
I
i
i
I
''"
(,.)~
~-~
"0 .
100,000
/
cd
o
z
z"
10,OOO
//
,//
Z
o
Z
8
/
1OOO
/
/
~J
100
/
/
L/
/
s
f
10O
100O
10,000
100,00O
1,O00,O00
TOTAL DISSOLVEDSOLIDS,IN MG/KG
Fig. 2. Concentration of major constituents relative to total dissolved solids in waters from
the Bolivian Altiplano from Lake Titicaca to Salar de Uyuni.
observations (J.P. Carmouze, pers. commun., 1979) indicate that the discharge of the Rio Desaguadero downstream from about midway between
Lake Titicaca and .Lake Popoo is presently dominated by inflow from the
Rio Mauri, which drains largely ignimbrite terrane of the Cordillera Occidental
to the west. Also, analyses made in 1976 of some h o t springs* in the region
of Calamarca, Oruro, and Challapata (Fig. 1) show primarily Na-HCO3 and
* Sampled by W.D. Carter, USGS.
62
Na--Cl waters. Increased inflow from these springs in the past would have
caused Na and chloride to constitute a larger share of the total solute composition of the lakes.
Despite inflow variations, the most obvious explanation for apparent loss
of dissolved carbonate and sulfate over the concentration range of the Altiplano lake waters would be the precipitation of calcite and gypsum. The
similar slopes of the Ca, Mg and SO4 increases between Titicaca and Poopo
actually reflect continuous loss relative to conservative total solute content.
The relative decrease in HCO3 content is to be expected when waters already
saturated with respect to calcium carbonate receive added input of Ca from
other sources. Indeed, Boulang~ et al. {1978) report up to 30% carbonate in
the sediments of central Lake Poopo, apparently a low-Mg calcite. Further
loss of Mg from solution may be associated with smectite clays or precipitation
of diatomaceous silica (Jones and Van Denburgh, 1966; Eugster and Jones,
1979).
The fundamental importance of calcite and gypsum precipitation to the
subsequent geochemical evolution of saline waters on the Altiplano has been
discussed previously (Risacher, 1978). Inasmuch as the northern Altiplano
region is an area of calcareous sedimentary rocks, molar Ca concentrations in
inflow of the area generally have exceeded carbonate, and the constant ionactivity product accompanying calcite precipitation should be maintained by
relative Ca increase as HCO3 is lost, until gypsum saturation is reached. Thereafter, calcium is rapidly depleted as sulfate increases at constant aCa" aso4 •
The result is the trend toward Na--(Mg)--C1--SO4 brines predicted by calculations (Risacher, 1978) on the freshwater compositions of the northern Altiplano.
Besides the trends in major solute composition relative to total dissolved
solids content shown in Fig. 2, changes in individual solute concentrations
can be shown with respect to chloride, which is normally conserved in solution over the complete concentration range of natural waters up to halite
saturation [for application to other hydrologically closed basins, see Eugster
and Jones (1979)]. Computations of the change in major solute matrix over
the drainage basin of Lakes Titicaca and Poopo, based on the data of Carmouze
et al. (1978), are given in Table I.
From Table I, it is immediately apparent that Na in the northern Altiplano
waters is as conservative as chloride, and this suggests that the two solutes have
a similar source. At the same time, Ca and HCO3 are the principal cation and
anion, respectively, lost in the compositional evolution from north to south,
as expected from calcite precipit.ation. The loss of proportionally more HCO3
than Ca at Lake P o o p o may be related to alkalinity loss accompanying cation
sorption with increasing salinity. Decrease in proportion of sulfate is significant
in the Lake Titicaca system at concentration levels below gypsum saturation,
whereas Mg proportions are notably decreased only on inflow to Lake Poopo.
The most unexpected figures of Table I are those for K. The proportion of K
lost on inflow to Lake Titicaca is nearly regained in outflow, and almost 20%
of the K proportion lost on inflow to Lake Poopo is regained in the more con-
63
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o
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o o o ~ o o ~
~ o o ~ o o ~
~
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I
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~o+
nO
0..~
I
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nO 0
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=
6
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64
centrated waters to the southwest. This is probably related to organic uptake
and release on decay or to concentration-dependent ion exchange with finegrained particulates.
Between Lake Poopo and the Salar de Coipasa the compositional trend for
the most saline waters continues toward chloride dominance (Fig. 2). Ca has
been decreased markedly, and SO4, though it continues to increase, does so by
a factor notably less than that for total solutes. The extent of the precipitation
of calcite and gypsum has become readily apparent in the abundant gypsum
crystals in the muds at the margin of the salar and deposits of algal limestone
along terraces of old lake levels. The proportional increase of Mg and K at
Coipasa is enhanced by the loss of Na to initial halite precipitation.
Between Coipasa and the Salar de Uyuni, solute compositional trends are
discontinuous like the present drainage and, though illustrating diverse changes
in individual constituents relative to the Lake Poopo chemistry, the TDS concentrations are very similar. From the limited data, Coipasa brines appear to
be proportionally higher in Na and lower in Mg and K--SO4 than the brines
from Salar de Uyuni. Na is definitely depleted in the most concentrated liquors
from both salars, whereas Mg, K and, to a lesser extent, SO4 are decidely enriched relative to the TDS concentration. Such trends are consistent with the
precipitation of halite from a solute matrix dominated by chloride [see solute
concentration trends for marine brines given by Carpenter (1978)].
CHEMICAL ANALYSES OF BRINES FROM THE GREAT SALARS
Table II is a compilation of the chemical analyses of brine samples: 18 from
central Salar de Uyuni, 15 from the Rio Grande de Lipez delta area, and 3 from
Salar de Coipasa. Fig. 3 is a map of the area showing sample locations. All
brines with a density greater than 1.2 g/cm 3 were essentially saturated in
halite. The solutes remaining were largely SO4, K and Mg.
Analytical methods were based on those of Rainwater and Thatcher (1960)
and Skougstad et al. (1979), subject to some of the same considerations noted
by Jones et al. (1977). Samples were diluted by a factor of 10 prior to determination of the major constituents. Chloride was determined gravimetrically,
and sulfate was determined turbidimetrically by stabilizing the BaSO4 suspension with gelatin as proposed by Tabatabai (1974). The major cations were
analyzed by atomic absorption spectrophotonmtry. Ca was analyzed in a
nitrous oxide--acetylene flame with the addition of K, and Mg was determined
in an air--acetylene flame with the addition of La. To allow for the matrix
effects in the analysis of K, Na was added to the standards to match the concentration of the samples. Li was determined by standard addition. It should
be noted that the values reported for bromide, analyzed by oxidation with
hypochlorite, have not been corrected for iodide, which we have considered
to be negligible. Because chloride is conserved in solution only to the point
of halite saturabion, the various constituents were initially considered with
respect to increasing density to explore trends in individual solute content
with the TDS concentration.
65
67oW
68oW
20°S l
123
sA'AR °E u °N' /31
~14o~"~'-4LL.
,o
}
°
"-"
'126,M.5
_
d)
J
.M17
M19
0
i
. . . .
25
1
. . . .
50 KM
i
Fig. 3. M a p o f t h e Salar de U y u n i illustrating t h e l o c a t i o n o f b r i n e s a m p l e traverses a n d
individual b r i n e s a m p l e collections.
The relationship of chloride, Na, K, M g and SO4 content to density indicate
that for brines denser than 1.20 g/cm 3 , chloride was nearly constant, with a
decrease of Na being balanced by increases in K, M g and SO4. The chloride
contents of the few Coipasa brines obtained were slightlyless than those from
Uyuni, even though their density was higher and the Na/K ratio at comparable
total solute content was greater. These compositional differences can be explained in part by the inflow history and present hydrologic balance of the
two salars. Uyuni is n o w a completely closed basin. It presumably has been
isolated from the Pleistocene lake systems for some time longer than Coipasa.
Inflow is n o w principally from the Rio Grande de Lipez with minor contributions from other small drainage basins to the south, and from groundwater
flow, none of which can be expected to contribute the major discharge or
66
T A B L E II
A n a l y s e s (in m g / k g , u n f i l t e r e d ; S.L. R e t t i g , analyst) o f brines f r o m the salt crust of S a l a r de U y u n i , R i o Grande
d e L i p e z d e l t a area, and Salar de Coipasa, Bolivia
pH
Density
( g / c m 3)
Salar de U y u n i Central Traverse:
100
7.40
1.204
101
7.34
1.207
106
7.23
1.210
108
7.29
1.210
109
7.03
1.221
113
7.22
1.213
115
7.41
1.205
117
120
122
126
129
131
133
146
HCO 3
SO 4
G1
F
Br
Ca
Mg
3,270
5,080
7,720
7,000
15,000
7,420
4,930
5,910
9,150
8,520
19,100
10,100
8,420
5,430
10,700
158,000
157,000
157,000
157,000
157,000
158,000
157,000
157,000
156,000
157,000
155,000
158,000
158,000
157,000
157,000
5.8
5.8
7.4
7.4
12
9.1
9.1
8.3
12
14
9.8
12
11
9.9
13
17
27
49
50
86
55
15
14
47
34
90
75
66
43
64
329
489
468
512
341
517
647
585
451
484
183
380
558
609
393
1,910
3,080
5,300
5,330
11,500
7,310
2,560
3,480
7,380
6,250
13,800
9,720
6,410
4,620
8,750
7.34
7.24
7.39
7.05
7.13
7.24
7.35
7.12
1.207
1.213
1.211
1.224
1.214
1.211
1.208
1.213
106
159
263
329
570
482
162
178
354
264
515
488
376
240
511
7.07
5.99
7.16
7.48
6.37
6.69
1.216
1.279
1.131
1.035
1.256
1.224
633
-270
300
1,800
735
13,400
46,500
5,850
959
32,600
18,100
157,000
160,000
103,000
30,600
161,000
155,000
8.0
26
3.0
2.0
17
4.0
69
291
40
18
210
96
366
80
1,550
679
0
133
9,530
41,300
5,480
502
36,200
12,300
1.000
1.000
1.019
1.093
1.050
1.143
1.151
1.164
1.213
1.210
1.198
1.215
1.211
225
303
326
448
589
610
800
1,100
816
744
893
1,000
566
300
600
2,360
4,680
5,430
3,150
4,950
6,570
10,500
9,180
13,000
13,000
8,430
875
3,380
12,300
72,600
38,200
113,000
117,000
126,000
157,000
157,000
143,000
1560000
157,000
1.0
1.3
4.9
6.4
6.7
7.0
6.1
8.6
6.6
9.9
10
13
12
<10
~10
(10
34
19
45
64
79
91
72
96
123
79
161
429
1,180
1,780
807
2,340
1,440
1,070
418
467
382
321
491
45
197
554
4,270
3,240
7,570
8,190
9,830
8,660
8,420
11,300
9,350
6,880
1.247
1.224
1.223
922
678
642
33,300
20,100
20,500
150,000
151,000
152,000
30
19
20
221
88
116
67
193
207
21,800
8,990
9,910
,~E crustal margins:
M.5
M-17
M-19
M-35
C o l c h a n i Salt Works
Isla de los P c s c a d o r e s
R i o G r a n d e de L i p e z Delta:
858
8.07
860
7.83
862
7.83
862A
7.00
863
7.56
864
7.05
865
7.13
866
7.57
867
7.32
869
7.30
871
7.21
873
7.10
875
7.34
S. d e Coipasa:
142
143
144
6.97
7.37
7.34
* T h o u g h n o particulate c o n t a m i n a t i o n w a s n o t e d visually, l a c k o f filtration suggests t h e s e values be c o n s i d e r e d
semi-quantitative.
quantities of dissolved material appropriate to the size of the salt crust. Whatever inflow waters reach the central salt crusts of the salar are quickly dominated in solute composition by re-solution of NaC1 from thick pre-existing
halite. As evaporation proceeds, any carbonate or sulfate is precipitated as
calcite, gypsum, or sodium sulfate, and on reprecipitation of NaCI, Mg and K
are enriched in the resulting chloride liquors. Therefore Uyuni has apparently
reached a final stage o f development, and any future changes in its final brine
67
Na
K
Sr
Li
B
PO 4
97,700
94°400
90,300
91,100
77,400
85,500
96,100
94,400
87,000
88,700
71,700
78,200
85,800
89,700
80,500
1,950
3,340
5,830
10,000
11,600
7,480
2,790
3,620
7,760
6,660
14,800
11,100
7,210
4,730
8.860
7.8
11
18
12
8.2
22
11
14
13
16
3.6
18
19
18
15
104
134
215
303
512
441
139
193
354
351
817
577
343
246
511
75
115
178
169
351
238
82
109
195
174
492
291
199
123
270
0.38
0.68
0.67
1.3
1.1
1.1
0.73
0.66
0.91
0.76
1.4
1.7
0.71
0.41
0.98
6.2
5.5
5.9
6.1
6.5
6.5
7.4
7.6
7.5
7.7
7.5
7.2
9.7
8.3
9.2
82,300
28,500
57,500
18,100
40,600
80,400
10,900
5.1
27,400
0.5
3,760
7.3
1,450
27
24,100 ~ 5 0
10,600 < 5 0
469
2,060
221
232
1,510
490
337
1,090
93
108
722
527
2.0
5.9
0.9
8.0
6.4
0.88
4.4
20
15
61
18
7.9
515
1,550
6,530
36,400
18,700
54,000
56,500
59,500
82,400
80,600
68,400
78,800
83,000
27
80
294
2,930
1,710
5,380
6,340
6,790
8,570
8,680
10,400
9,050
6,940
2.5
5.8
23
32
21
37
21
12
9.1
9.5
6.1
3.4
9.5
3.1
10
26
217
142
503
548
579
459
445
611
521
353
10
21
31
229
110
421
727
469
317
304
423
516
235
0.66
0.51
0.54
0.23
2.2
2.8
7.6
3.3
1.7
1.4
2.0
2.1
1.2
47
51
60
43
61
29
12
1.9
14
5.4
1.8
1.8
14
59,600
83,500
82,100
18,300
7,140
7,420
18
14
18
524
251
276
1,300
497
561
13
6.1
7.7
SiO*
20
4.9
5.6
composition will be an increase in Mg and K at the expense of Na. On the
other hand, Coipasa has continued to receive inflow (presumably high in Na)
from Lake Poopo for a longer period during the desiccation of the Pleistocene
lakes, and thus has not proceeded as far in its geochemical evolution. In addition, recent satellite observation suggests surficial brine levels are presently
sustained by discharge of inflow from the north. Eventually Coipasa might be
expected to achieve brine compositions similar to those in central Uyuni.
68
S O L U T E C H E M I S T R Y vs. C O N C E N T R A T I O N
Because the relation between the TDS content and density varies as a function of the solution composition, the effect of evaporative concentration on
individual constituents in solution must be examined by reference to a conservative solute.
Chloride and bromide
Chloride is normally appropriate for such purposes (Hanor, 1977; Jones et al.,
1977; Eugster and Jones, 1979), but in view of the halite predoininance in
the Altiplano salars, another element must be considered. Experience with
bromide as a reference in marine evaporites (Braltsch, 1971) suggests its use
here. Although a small amount of Br is taken up in solid solution in precipitating halite, the fractionation strongly favors the residual solution (Braitsch
and Hermann, 1963).
The relation of bromide to density for brines from Uyuni and Coipasa is
given in Fig. 4. The sample points are most readily divided into two distinct
groups: brines from the traverse across the central Salar de Uyuni, including
brine collected at the margin of the Isla de los Pescadores in the middle of
the salar, and those from the Rio Grande de Lipez delta area associated with
the continuous halite crust of the main salar; and the majority of samples
from the Rio Grande delta plus highly concentrated residual brines from the
southeast Uyuni crustal edge and the Colchani salt works. The latter group
220
200 -
eoipasa.
/+
~M17I
Colcha--
× Uyuni traverse
• Rio Grande Delta
n~/
~i
160
120
1
•
!
~
i
/.
80~-
" :Y2
.'c°'oa'a
i!
;
1
40 k
k-- -
.
-
O[ _ L _ _ . ~
100
1.04
-!
I
J .... 1~__ t
1.08
1.12
I__..L_
1.16
DENSITY
I
L___I__
1.20
I
1.24
i
L
1.28
t.30
Fig. 4. Bromide concentrations of brines f r o m Salar de U y u n i p l o t t e d against density.
Location of samples is shown in Fig. 3 and analytical data are given in Table II.
69
can be fit to a p o w e r function with r = 0.93 (Table III), which suggests a
single regular fractionation process accompanying continuous solution concentration. In contrast, the wide range of the bromide concentrations associated
with the nearly constant chloride content and the narrow density range
(1.20--1.23 g/cm 3) of the Uyuni crustal brines most probably reflects variable
or cyclic surficial halite re-solution, resulting in progressive depletion of Br in
associated fluids. The highest Br values from central Uyuni are associated with
the brine pools described by Ericksen et al. (1978), and may represent upwelling of denser residual brine from the lower crust along major fractures.
The Salar de Coipasa brines fit the general relationships established for Uyuni.
These relatively straightforward correlations appear to justify the use of bromide
as a reference for delineation of processes affecting other solutes.
TABLE III
Regression parameters for interelement correlation -- Brines of the Salar de Uyuni;
Rio Grande de Lipez delta area, and Salar de Coipasa
Samples
Central Uyuni
+ Isla de Pescadores
Rio Grande delta
area + Colchani
Central Uyuni traverse
Central Uyuni traverse
+ Colchani and SE Uyuni
crust edge concentrate
Central Uyuni +
Isla de Pescadores
All samples
Central Uyuni traverse
All samples
Central Uyuni traverse
All samples
Uyuni traverse +
Colchani + SE crust
Rio Grande delta
+ Colchani + SE crust
Uyuni traverse
All Uyuni samples
Uyuni traverse
Central Uyuni +
Colchani + SE crust
All samples
*Linear:
y ffi a + b x ;
Elements
Expression*
Br, density
linear
1.20
Br, density
Na, Br
power
linear
0.85
313.0
0.076
-0.003
0.93
0.89
Na, Br
linear
379.0
-0.0038
0.97
Mg, Br
linear
3.74
0.0070
0.87
Mg, Br
K, Br
K, Br
SO4, Br
SO4, Br
linear
linear
linear
linear
linear
12.0
6.15
- 5.49
5.92
5.97
0.0067
0.0058
0.0095
0.0050
0.0059
0.89
0.82
0.83
0.73
0.90
Li, density
linear
1.20
Li, density
Li, Br
Li, Br
B, Br
power
linear
linear
linear
0.814
10.00
4.88
8.03
0.059
0.111
0.137
0.199
0.74
0.79
0.92
0.83
B, Br
B, Br
linear
linear
-4.03
7.28
0.259
0 .1 9 5
0.94
0.82
power: y = ax b.
a
b
r 2 (correlation coeff.)
0.0002
0.85
3 . 7 . 1 0 -5 0.97
70
Sodium
The most graphic illustration of the development of two distinct brine types
at Salar de Uyuni is seen in the Br vs. Na relation concentration in solution
(Fig. 5). Brines associated with the halite crust of the salar show a systematic
increase in Br associated with a decrease in Na (and concomitant increases in
Mg and K). Indeed, without much change in slope, this trend can be extended
to include the most concentrated solutions analyzed from the area. However,
samples from the Rio Grande delta which have not been in contact with the
halite crust of the salar show a strong increase in Na with increasing bromide
to the point of intersection with the trend of central Uyuni crustal brines and,
apparently, saturation with respect to halite*, as indicated by a relatively constant chloride content (see Table I). The change in slope of Na increase at
~ 5- 104 mg/kg may be related to ulexite (NaCaBsO9 • 8H20) precipitation.
At higher TDS concentrations, Na is replaced as the major cation by increasing amounts of K and particularly Mg, as mentioned earlier. Because of
the predominance of chloride over Na at the onset of halite precipitation,
further concentration forces a decrease in Na levels at nearly c onstant chloride
content. This effect is enhanced by the proportionally greater increase in the
activity coefficient of Na ÷ over C1- o n further concentration. These relations
i
× ,~
• Uyuni traverse
• Rio Grande Delta
Coipasa
~'~'~ i ' ' Z de
80,000~--
60,000'~
z
•
• .
•
•
~
•
I
~.~/~
•
Cot;oasa
i
-.~,~
!
40,000~"~--
M1Z -4
i
20,000!-
I
40
I
80
120
160
mg/kg Br
200
240
280 300
Fig. 5. S o d i u m concentrations o f brines from Salar de U y u n i plotted against bromide
content. Location o f samples is s h o w n in Fig. 3 and analytical data are given in Table II.
*Halite saturation indices -- log of the ratio o f the activity product to equilibrium constant
at temperature o f collection -- are within 0.15 units or less o f equilibrium, as calculated from
the c o m p u t e r program WATEQF ( P l u m m e r et al., 1976), but U y u n i brine concentrations
are b e y o n d the theoretically prescribed application o f the aqueous model.
71
further substantiate the subdivision of Uyuni brine compositions into primary
and re-solution types, as suggested in the plot of bromide vs. density. On the
other hand, the limited data on Coipasa brines indicates that they are proportionately higher in Na, especially at a higher concentration, and somewhat
lower in chloride than those from Uyuni. As mentioned earlier, this may have
resulted from a higher proportion of sodium salts in inflow to Coipasa since
drainage separation of the salars.
Magnesium
Mg appears to offer a good correlation with bromide over the entire concentration range of all salar brines examined (Fig. 6). The regression line derived
for Mg vs. Br contents of brines from the central Uyuni salt crust has a nearly
identical slope to that obtained for Mg vs. Br in all samples analyzed. However,
significant departure from a simple evaporative concentration reference line
indicates a continuous loss of Mg from solution as total solutes increase. Specifically, the Mg loss seems to have been greater in the lower Rio Grande delta
than at central Uyuni, and still greater at Coipasa. Without further information
on associated mineralogy or ion balances, one can only speculate whether this
may be due to uptake by carbonate or silicate (Jones and Van Denburgh, 1966;
Garrels and MacKenzie, 1967; Hardie and Eugster, 1970; Eugster and Hardie,
1978; Eugster and Jones, 1979).
50,000
40,000
I
I
I
I
Uyuni traverse
• Rio Grande Delta
I
I
I
I
I
I
I
I
_
I
I
M+~
30,000 --
//
io,ooc
,, i ~ 2 , ~ .
/ ~)e~/
/.
/ ...~¢
0(~
40
80
120
160
mg/kg Br
200
240
280 300
Fig. 6. Magnesium concentrations of brines from Salar de Uyuni plotted against bromide
content. Location of samples is shown in Fig. 3 and analytical data are given in Table II.
Potassium
K shows a relation to bromide similar to that for Mg overall, with some
subtle b u t significant differences (Fig. 7). For the brines of the central Uyuni
72
50.000~
[
!
~
•
•
~
]
!
]
T
i
[
'.
T
l
T
× Uyuni traverse
• Rio Grande Delta
40 OOOP
S
//
30,000 ~
2
~,~ , "
. J
Colchan
I ~-
M 17"
20,000
10000-
0
,
/"
40
~
•
•
80
de O
•
120
160
mg/kg Br
i
200
240
280300
Fig. 7. Potassium concentrations of brines from Salar de Uyuni plotted against bromide
content. Location of samples is shown in Fig. 3 and analytical data are given in Table II.
salt crust, the slope of the K vs. Br regression is close to that for Mg vs. Br,
b u t the departure from the slope of simple evaporative concentration is greater
The relative loss of K from solution is even more pronounced when all samples
are taken into account, and the deviation from the central Uyuni trend is apparent, especially for the Coipasa samples. In fact, the Rio Grande delta-Coipasa data for both Mg and K vs. Br describe trends that could be examples
of Eugster and Jones' (1979) type-IIa curve for the dominant ion of a binary
precipitate, or the type-IV curve of sorption or exchange, or combinations.
The distinct differences seen in the highest total solute concentration brines
from the Colchani salt works, Coipasa, or the SE Uyuni salt crust edge area
suggest that more than one process is involved. Indeed, sylvite has been identified in the salt crusts at Uyuni (F. Risacher, unpublished data, 1978), indicating that simple salt precipitation is operative in addition to the commonly
presumed uptake by clays.
Sulfate
The plot of SO4 vs. Br shows the most clear-cut separation of the central
Uyuni salt crust brines from the fluids of the Rio Grande de Lipez delta area
(Fig. 8). Although the trend of sulfate increase with increasing bromide (and
total solute concentration) for both data sets falls below the slope for straightforward evaporative concentration, the Rio Grande delta brines appear progressively more depleted in SO4 than the central Uyuni crustal brines. This
also appears to be the case for the most concentrated brines from Colchani,
Coipasa and the SE Uyuni crustal edge (e.g., M-17, Fig. 3), though not as
markedly. Both data sets appear to offer an example of Eugster and Jones'
73
50,000
I
I
I
]
Uyuni traverse
• Rio Grande Delta
I
I
I
I
]
.'i
I
I
]
./"
] j
J
/."
J
/
~ + . Co,'pa=a
co,c...,
•
M17
40,000
o'..
,#,,,
C~'30,000
#,z
20,000
." x7a
,,"" //x
7
T de PP.
=.=.-
10,000
"I
I
40
I
i
80
I
I
120
I
mg/kg
I
160
Br
I
I
200
I
I
240
I
I
280 300
Fig. 8. Sulfate concentrations of brines from Salar de Uyuni plotted against bromide content. Location of samples is shown in Fig. 3 and analytical data are given in Table II.
(1979) type-III curve for a continuous concentration independent process.
Bacterial reduction is the most obvious mechanism to provide progressive
depletion of sulfate, following gypsum precipitation. Sulfur species reduced
in the fine muds of the Rio Grande delta area or beneath the central halite
deposits are probably re-oxidized to some extent at the surface, especially in
the course o f crustal re-solution by oxidized surficial runoff waters, resulting
in less net depletion in the central Uyuni crustal brines than in the principal
delta area. In the most concentrated brines, sulfate proportions increase as a
consequence of the limit on chloride increase imposed by halite precipitation,
and, to a much lesser extent, because of the decreased solubility o f CaSO4
with increased total salinity.
Lithium and boron
The elements of perhaps the greatest current economic interest, the minor
solutes Li and B, which have been particularly associated by Risacher and
Miranda (1977) with the volcanic terrane south of Uyuni, also provide insights to processes affecting the geochemical evolution of the salar brines.
With respect to density, the Li values show definite similarity to the relationship developed for bromide, whereby brines with densities between 1.20 and
1.23 g/cm 3 associated with the continuous salt crust are apparently lower in
Li than other samples with either higher or lower densities (Fig. 9). The brines
from the Salar de Coipasa have particularly l o w Li contents, suggesting lack
of source. Although n o t as satisfactory a correlation as obtained for Br, a
p o w e r equation with r = 0.78 can be fit to Li vs. density values for the more
74
4000r
- r
;
•
[
]
l
r
.
[
;
[
]
i
i
3000~
\.
l
, Uyuni traverse
• Rio G r a n d e D e l t a
/
2000
~)¢~olchanl
lOOOi
•
•
o ~
1.00
J _ ~
1.04
1.08
~
e~° ~_
~
.Coipasa
4~ ,Coipasa
1
1.12
.t
I
1.16
•
~
1.20
1
. L
124
J
•
1.28
.;
130
DENSITY
Fig.9. Lithium concentrations of brines from Salar de Uyuni plotted against density.
Location of samples is shown in Fig. 3 and analytical data in Table II.
dilute (density < 1.20 g/cm 3) samples from the Rio Grande delta plus the
bitterns of Colchani and the SE Uyuni crustal edge. With the exception of
the sample from one of the central brine pools described by Ericksen et al.
(1978), crustal brines of the Salar de Uyuni contain half or less the Li predicted
by the power equation. In contrast, the most highly concentrated non-crustal
brines of the Rio Grande delta contain even more Li than indicated by the
equation (Risacher, 1978).
The Li vs. Br content relation (Fig. 10) suggests a tenuous explanation for
the previous results. The Li vs. Br regression slope for the central Uyuni crustal
brines is nearly parallel to the evaporative concentration trend, indicating that
Li is basically conserved in these fluids. At the same time, a few less concentrated
samples from the Rio Grande delta contain unusually high amounts. An apparent limit to Li enrichment is reached at about 600 mg/kg in the presence o f
mud and/or thin crust. This limit is probably related to uptake by smectite clays.
Some such mechanism has apparently been very effective at Coipasa; the low
slope prescribed by its three points cannot be related to lack of source alone.
Similar, though less pronounced, effects have been noted previously for both
Mg and K.
From the B vs. Br content relation (Fig. 11) it is apparent that, as with Li,
a few less concentrated brines from the Rio Grande delta fit a relation close to
that for simple evaporative concentration. However, the data from the majority
of brines from the Salars de Uyuni and Coipasa, including the most concentrated
fluids, suggest a long-term depletion of B from solution. The ulexite deposits in
the Rio Grande delta area and disseminated crystals in the muds elsewhere
readily account for precipitation of B from the brines (Risacher, 1978).
75
2200 - 2000 -
I
I
I
I
Uyuni Traverse
• Rio Grande Delta
I
I
I
I
I
,¢. / ' "
~1200 -
O°,O,°:
• /
,/~,,'.~
/
0
40
I
I
I
I
jT'M/7~
1600 -
4eeL
I
° •'z
/~/
/ Colchani~
~6~9
~/
=Coipasa
- ......
~e P
".
.Ce,,asa
80
120
160
mg/kg Br
200
240
280 300
Fig. 10. L i t h i u m concentrations o f brines from Salar de U y u n i plotted against b r o m i d e
content. Location o f samples is s h o w n in Fig. 3 and analytical data in Table II.
2000
I
I
1
I
× Uyuni traverse
• Rio Grande Delta
I
I
l
I
I
I
I
I
I --
1800
•~o~,'
=
==
asa~
o% /
J
M17.
°s~
#~e/,"
_
see
.........
/ • ~, /
_
400
6
~ ' ~ ' ~ ...........
U°"
~
_
+ Colchani
-~"
//
Or,~
0
•
"
•
40
~
[ ו i
80
I
120
L
I
160
mg/kg Br
I
I
200
I
I
240
I
I
280 300
Fig. 11, Boron concentrations o f brines from Salar de U y u n i plotted against b r o m i d e
content. Location of samples is s h o w n in Fig. 3 and analytical data in Table II.
As compared with the other solutes referenced to conservative Br, the Li
and, to a lesser extent, B contents o f the Salar de Uyuni brines appear to be
less affected by the more complex processes and potential mechanisms of loss
from solution than the major solutes. The c o m m o n association of the t w o
elements has suggested use of borate deposits as a prospecting tool for Li-rich
76
solutions. Consequently, Li and B concentrations of dilute water from Lake
Titicaca and streams tributary to the Salar De Uyuni from thel south were
plotted against TDS (Fig. 12). The results indicate that drainages such as the
Rio Grande de Lipez established in relatively fresh volcanic (rhyolitic) terraine
carry Li and B together in direct and relatively high proportion to total solutes,
whereas other streams may contain much B without correspondingly high
levels of Li. A contrast may also be seen in the relations of the two elements to
bromide for the brines. Li levels at Coipasa are notably lower than at Uyuni,
whereas B contents are similar for a given Br content.
3.0 - -
I
]- . . . . . . . . . . . . . . . . T . . . . . .
I
~
• Lithium
o Boron
-
•
- - ~
]120
~,R. Grande de Lipez
.i
',
10 0
~ i s.o
2.0l
R. Quebrada
z
,
R. Puca Mayu ,I ~ 6 0
Chaqui Mayu
z_
z
E
~ R. San Antomo
-
~
i
10-
6
•
i,~O
dltute small streams $,
tributary to Uyuni ~ R' Alota
~,o
0 '
"2
6
~ L.
i
Titicaca
i
tl
0.0
I,LI
-
I
506
1000
___
I
1500
2000
DISSOLVED SOLIDS, IN MG/L
I
2500
I
3000
-
Joo
Fig. 12. Lithium and boron concentrations plotted against dissolved solids for Uyuni inflow.
The Rio Grande de Lipez is the largest inflow stream; its tributaries include the Rio San Antonio and Rio Alota. The Rio Quebrada Chaqui Mayu, and Rio Puca Mayu flow directly
into Uyuni from the west and east of the Rio Grande, respectively. Lake Titicaca is shown
for comparison.
Stron tium
Sr concentrations in the central Uyuni crustal brines appear to bear no relation to total solute content or any other specific constituent, ranging from
3.6 to 22 mg/kg for densities_between 1.20 and 1.23 g/cm 3. This scatter may
be the result of variability in the recrystallization of sulfate or carbonate
minerals bearing Sr in solid solution. The Sr contents of brines from the Rio
Grande delta, however, show a very interesting trend with increasing density
(Fig. 13). Sr content increases to a m a x i m u m of 37 mg/kg at a density of 1.143
g/cm 3. At higher densities the Sr concentration decreases abruptly in what
appears to be a logarithmic manner to minimum of 0.5 mg/kg at a density
of 1.279 g/cm 3 in the SE Uyuni crustal concentrate. This abrupt decrease
suggests the precipitation of a St-bearing mineral; from current thermodynamic data, the most likely phase seems t o be celestite.
77
50--
1
1
1
I
I
I
I
I
I
I
I
I
I
I
x Uyuni traverse
• Rio Grande Delta
4O
3O
x
Coipasa
2O
Z,./
• Coipasa
×
x
~,
x
10x
1
1.00
I
1.04
]
]
1.08
I
I
1.12
I
I
1.16
DENSITY
I
I
1.20
×
I
I
1.24
L
dM1
1.28
1.30
Fig. 13. Strontium concentrations of brines from Salar de Uyuni plotted against density.
Location of samples is shown in Fig. 3 and analytical data in Table II.
SUMMARY
Examination of recent hydrochemical data from the Bolivian Altiplano has
outlined the compositional trends and principal processes leading to the
present chemistry of brines in the largest playa in the world, the Salar de Uyuni.
These trends have been delineated with respect to total solute concentration
in the succession of remnants of two very large lakes, connected by overflow,
which occupied much of the floor of the Altiplano during the Pleistocene.
They have illustrated the effects of evaporative concentration, the widespread
precipitation of calcite, gypsum and halite and the development of a residual
K--Mg--CI--(SO4) brine in a continental environment. Although the basic
trends were already established in the geochemical evolution of the present
hydrologic system between Lakes Titicaca and Poopo, the greatest constituent
percentage change has been shown to be between southwest Lake Poopo and
the surficial brines o f the Salar de Coipasa.
Composition trends have also been worked out for brines from within the
area of the Salar de Uyuni and compared with Coipasa, utilizing the conservation of bromide in the fluid phase during evaporative concentration. This has
permitted the differentiation of truly residual from re-solution brines, particularly from the relation of Br with Na, Similarly, reference to Br has indicated the sorptive or precipitate loss of K, Mg and Li, especially at Coipasa,
and the reductive loss of sulfate. It has also demonstrated the relative conservancy of these elements in solution in contact with salt crust rather than fine
sediment. At the same time, B appears to be influenced by a similar precipita-
78
tion process at both places. Precipitate control on Sr concentration is suggested
i~ one part of the Uyuni system.
The Bolivian Altiplano has presented a unique opportunity to look at the
geochemical evolution of closed-basin waters. Though the precipitation of
calcite, gypsum and halite, and resulting trends might have been predicted,
the Altiplano area has provided insights into the importance of related processes in a major hydrologic system.
ACKNOWLEDGEMENTS
Publication authorized by the Director, U.S, Geological Survey. We wish
to thank W. Back, D.C. Thorstenson and H.P. Eugster for review of the manuscript, and the staff of GEOBOL for their support during the field investiga~
tions. We are particularly grateful to George Ericksen of the U.S. Geological
Survey for initiating this particular effort, and to him and Hans Eugster for
continued advice and encouragement throughout the course of the work.
We also thank the reviewers of this Journal, J.I. Drever and A.J. Ellis, for their
comments.
REFERENCES
Ahlfeld, F. and Branisa, L., 1960. Geologic de Bolivia. Instituto Boliviano de Petroleo,
Don Bosco, La Paz, 245 pp.
Boulang6, B., Rodrigo, L.A. and Vargas, C., 1978. Morphologie, formation et aspects s6dimentologiques du lac Poopo (Bolivie). Cah. O.R.S.T.O.M., S6r. G6ol., 10(1): 69--78.
Braitsch, O., 1971. Salt Deposits: Their Origin and Composition. Springer, New York,
N.Y., 297 pp.
Braitsch, O. and Hermann, A.G., 1963. Zur Geochemie des Broms in salinaren Sedimenten,
Teil I. Experimentelle Bestimmung der Br-Verteilung in verschiedenen naturlichen Salzsystemen. Geochim. Cosmochim. Acta, 27: 361--391.
Carmouze, J.P., Arze, C. and Quintanilla, J., 1978. Circulation de materia (agua-sales
disueltas) atraves del distema fluvio-lacustre del Altiplano: la regulacion hidrica e hidroquimica de los lagos Titicaca y Poopo. Cah. O.R.S.T.O.M., S6r. G6ol., 10(1): 49--68.
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